The proposed work will include both a genetic analysis of radiation mutagenesis and an examination of DNA replication in irradiated cells of the simple eukaryote, Saccharomyces cerevisiae. Previous work suggests that the RAD6 gene plays a key role in the formation of mutations, and is also concerned with a variety of error-free repair processes. The different functions of the RAD6 gene will be dissected by isolating new rad6 mutations and also non-translational suppressors of those mutations, which suppress some but not all aspects of the rad6 mutant phenotype. At the same time new mutations will be isolated that specifically block certain kinds of mutagenesis, for example the production of frameshift events. The relation between mutagenesis and the restoration of DNA synthesis in irradiated cells will be examined by studying the molecular weight, using sucrose gradient sedimentation, of newly synthesized DNA in a variety of mutant strains, deficient in various aspects of RAD6 dependent repair. DNA fiber autoradiography will also be used for this purpose, in order to separate effects of irradiation or chain initiation and elongation.